Method and apparatus for low-pressure pulsed coating

ABSTRACT

A coating material is deposited on a substrate by vacuum or low-pressure pulsed detonation coating. A detonation chamber receives a detonable mixture containing a coating precursor. The detonable mixture is ignited to produce detonation products laden with the coating precursor. The detonation products are accelerated in a low-pressure or vacuum chamber and discharged through a nozzle into contact with a substrate situated in low pressure to produce a high quality coating.

FIELD OF THE INVENTION

[0001] The present invention is directed to coating technology and, moreparticularly, to pulsed detonation coating.

DESCRIPTION OF RELATED ART

[0002] Several techniques have been used to implement thermal spraycoating. One approach has been High Velocity Oxygen/Fuel System (HVOF),in which solid particles are injected in high velocity gas produced byreaction of oxygen and a fuel at high pressure. Such systems typicallyare used for deposition at atmospheric pressure and primarily are usedfor coating metal alloys and WC/Co powders with particle sizes largerthan about 10μ. Other thermal spray coating techniques include plasmaspray, in which particles are heated and accelerated by high temperatureplasma produced by an electric discharge in an inert gas atmosphere.Plasma spray systems have been used for both atmospheric- andlow-pressure coatings.

[0003] Plasma coating and HVOF coating suffer from severaldisadvantages. One disadvantage is the inability of directly usingnanosized powder, or even small micron size (<10μ) particles. This isdue to the small particles closely following the streamlines of thecarrying gas and decelerating significantly in the stagnation region ofthe jet/substrate interaction. The decelerated particles are susceptibleto being diverted by the flow in the stagnation region, and are notdeposited on the substrate. To help alleviate this problem, nanosizedpowders typically are post-processed to create 10-60μ agglomerates thatretain nanostructure and are strong enough to survive the jetenvironment before deposition. However, agglomeration not only adds toprocessing cost, but also can promote grain growth, contamination, andother deterioration of the original powders.

[0004] Thermal spray coating also has been implemented by intermittentdetonations, e.g., by the use of a detonation gun (D-Gun). D-guns can beused for coating a wide variety of materials, such as metals, cermets,and ceramics. D-guns typically have a relatively long (often about 1 m),fluid-cooled barrel having a small inner diameter of about one inch.Typically, a mixture of reactive gases, such as oxygen and acetylene, isfed into the gun along with a comminuted coating material in two phases.The reactive gas mixture is ignited to produce a detonation wave, whichtravels down the barrel of the gun. The detonation wave heats andaccelerates the coating material particles, which are propelled out ofthe gun onto a substrate to be coated.

[0005] The detonation wave typically propagates with a speed of about2.5 km/sec in the tube and can accelerate the particle-laden detonationproducts to a velocity of about 2 km/sec. However, coating particlesnever reach the velocity of detonation products due to inertia. Inpractice, particle velocities usually are lower than about 900 m/sec.The temperature of the detonation products often reaches about 4000 K.After the coating material exits the barrel of the D-gun, a pulse ofnitrogen typically is used to purge the barrel. Newer designs of theD-guns allow operation frequencies of up to about 100 Hz. See, e.g., I.Fagoaga et al., “High Frequency Pulsed Detonation (HFPD): ProcessingParameters” (1997).

[0006] One example of a gas detonation coating apparatus is illustratedin U.S. Pat. No. 4,669,658 to Nevgod et al. A barrel enclosed in acasing has annular grooves made on an inner surface of an initialportion thereof. A main pipe housing a spark plug and having annulargrooves on its inner surface is inserted into the initial portion of thebarrel. In operation, a gas supply means is turned on. The apparatusworks in cycles, each cycle accompanied by gas flowing into the barreland the main pipe through tubes, gas conduits, and additional pipes.After the gases fill the barrel, the gas mixture is ignited in eachcycle with the aid of the spark plug. The detonation products are saidto quickly heat up the walls of the barrel and the annular grooves.

[0007] According to Nevgod, the gases flowing into the barrel are heatedup in two stages. During the first stage the gases are warmed up in theadditional pipes heated up in cycles by the detonation products. Theheat insulation tubes are said to prevent the pipes from cooling down.During the second stage, the gases are heated up in the barrel andpartially in the main pipe. The annular grooves on the inner cylindricalsurface of the initial portion of the barrel, the inner surface of themain pipe and on the inner surface of the cover on the end of thebarrel, are said to enhance the efficiency of heat exchange with thegases due to an increase in the heat exchange area and due to gasturbulization. The gases are heated to a temperature approximating thatof self-ignition. A plurality of ignition sites is provided toaccelerate the burning process.

[0008] Presently available detonation coating technology suffers fromseveral drawbacks. One major drawback is that the D-guns are bulky, withthe barrel alone often being as much as 1 m in length. Because of thedifficulties associated with handling the bulky D-gun, the D-gun oftenis held stationary while the substrate to be coated is moved relative tothe barrel of the D-gun. This is especially problematic for coatinglarger-sized articles that cannot easily be moved. Another drawback isthat the coating rate is limited by the relatively low operationfrequencies. Increase of operation frequency is possible if a shorterbarrel is used. However, a shorter barrel leads to decrease in particlevelocity, due to shorter cycle time available for particle accelerationby detonation products, which in turn will reduce coating quality.Reduction of particle size to less than 10μ will help particleacceleration by high-speed detonation products, however these particleswill quickly decelerate in the stagnation layer when approaching thecoated substrate, and thus will arrive to the substrate at low velocity.

[0009] It would be desirable to develop thermal spray coating technologythat enables the use of a smaller coating apparatus, especially one thatcan be adapted for coating the insides of tubes and otherdifficult-to-reach areas. Because for many applications coating qualityimproves with reduction of grain and particle size and with increase ofparticle impact velocity, it would be desirable to directly coatsmall-micron, sub-micron, and nanoscale particle that impact thesubstrate at very high velocities. It would be desirable to directlycoat small-micron and sub-micron sized particles at very high velocitiesto give high quality cold coatings or impact coatings. It would bedesirable to minimize the amount of local heating of the substratesurface during coating to enable the coating of very thin and/or lowmelting point substrates.

SUMMARY OF THE INVENTION

[0010] The present invention is directed to a method and apparatus forproducing a coating on a substrate situated in vacuum or low pressure bypulsed detonation coatings. A pulsed detonation gun comprises adetonation chamber having ignition means and an outlet nozzle. In onepreferred embodiment, a detonable mixture containing the coatingprecursor is formed in the detonation chamber. The detonable mixture isignited to produce detonation products containing the coating precursor.Following detonation, the detonation products containing the coatingprecursor particles are discharged through the nozzle and expand at highvelocities in a vacuum or low-pressure chamber. The coating precursorparticles are heated and accelerated toward a substrate to produce ahigh quality coating.

[0011] According to another preferred embodiment, a suspension of acoating precursor in a detonable fuel is injected into a detonationchamber to form a detonable mixture. The detonable mixture is ignited toproduce detonation products containing the coating precursor. Thedetonation products containing the coating precursor are discharged fromthe detonation chamber and accelerated in a low-pressure chamber. Thedetonation products containing the coating precursor are contacted witha substrate to produce a coating on the substrate.

[0012] According to an alternative embodiment of the invention, adetonation chamber has a first region containing ignition means and asecond region having a nozzle at an end portion thereof. A detonablemixture is injected into the first region of the detonation chamber. Acoating precursor, which can be provided in an inert gas or in asuspension, is injected into the second region of the detonationchamber. The detonable mixture is ignited, creating a wave front thataccelerates the coating precursor through the nozzle into a low-pressurechamber and into contact with a substrate to produce a coating.

[0013] The vacuum or low-pressure environment of the present inventionprovides a greater pressure gradient for the detonation products, e.g.,compared to conventional pulsed coating processes performed atatmospheric conditions. The greater pressure gradients yield higherparticle acceleration and velocities, which translate into the abilityto reduce the overall size of the apparatus and improve coating quality.The apparatus of the present invention thus can be made substantiallysmaller in size than the conventional, bulky D-guns. The apparatus canbe made portable and can be easily manipulated to accurately applycoatings to substrates having a wide variety of sizes and shapes.Maintaining low pressures near the substrate enables coatings of smallerparticles, e.g., small micron or nanosized particles, by avoidinginterference from the carrying gas, which will be at low pressure andlow density at the substrate.

[0014] Local heating of the substrate can be reduced or substantiallyavoided by reducing the duration of the detonation period, i.e., thetime of exposure to the high temperature detonation products.Preferably, any appreciable heating of the substrate is restricted to avery thin surface portion thereof. Optionally, the substrate surface canbe subjected to rapid temperature quenching after each exposure tofurther minimize local heating. Reduced local heating advantageouslyenables the coating of very thin and/or low melting point substrates andpermits higher coating rates.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will now be described in more detail withreference to preferred embodiments of the invention, given only by wayof example, and illustrated in the accompanying drawings in which:

[0016]FIG. 1A is a schematic illustration of a pulsed detonation coatingapparatus in accordance with a preferred embodiment of the invention;and

[0017]FIG. 1B and FIG. 1C illustrate a coating cycle for the coatingapparatus illustrated in FIG. 1A. FIG. 1B illustrates ignition anddetonation of a detonable mixture in a detonation chamber. FIG. 1Cillustrates detonation products laden with a coating precursor expandingin a vacuum chamber.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The coating process and apparatus of the present invention hasutility in applying a wide variety of coating materials to a widevariety of substrates. By way of example, nanoscale ceramic particlescan be coated onto lightweight, low melting point metals used inaircraft structures to impart erosion and corrosion resistance.Lightweight composite materials can be coated with hard ceramic or metalparticles to increase abrasion resistance. Another example is coatingbattleship metal surfaces with tough and hard ceramic nanoscale coatingsto eliminate the need for painting and reduce the amount of labor andmaterial needed for maintenance.

[0019] With reference to FIG. 1, an exemplary pulsed detonation coatingapparatus (1) of a preferred embodiment of the present inventionincludes a detonation chamber (10) for receiving a detonable mixture(55) containing one or more coating precursors. The detonation chamber(10) has a nozzle (100) for discharging detonation products laden withthe coating precursor(s) toward a substrate (200). An oxygen valve (5)and a fuel valve (15) are provided for controlling flow of oxidizer andfuel, respectively, into the detonation chamber (10). Examples of fuelsthat can be used include, but are not limited to, those detonable inmixtures with oxygen such as hydrogen, methane, propane, acetylene, orpropylene. Also, detonable mixtures of liquid fuels and oxygen can beused, e.g., kerosene/oxygen, alcohol/oxygen, benzene/oxygen and othersimilar mixtures. In addition, some detonable monopropellants can beused, such as nitromethane, nitroglycerin, or similar single-componentfuels that can be detonated. Selection of a suitable fuel will beapparent to persons skilled in the art and forms no part of the presentinvention.

[0020] The term “detonable mixture,” as used herein, refers to thecomponents present in the detonation chamber at the time of detonation.One example of a detonable mixture is an oxidizer, a fuel detonable inmixtures with oxygen, and a coating precursor. Another example of adetonable mixture is a monopropellant and a coating precursor. Asanother example, when a coating precursor is used as a fuel, a detonablemixture can comprise the detonable coating precursor and an oxidizer.

[0021] The coating precursor can comprise, by way of example, particlessuch as metals, cermets, ceramics, or combinations thereof. Non-limitingexamples of metalorganic compounds that can be used include silane,disilane, germane, tungsten hexaflurade, trimethylboron, cadmiumacetate, magnesium ethoxide, tantalum V-methoxide, tungsten V-ethoxide,zinc naphenate, and zirconium n-butoxide. Many coating precursors, suchas the aforementioned metalorganic compounds, can also function as adetonable fuel.

[0022] The size of the coating precursor particles can vary over a widerange. Often the mean particle size is about 50μ or less. Smaller micronparticle sizes also can be used, such as those having a mean particlesize of less than about 20μ or 10μ. In one embodiment of the invention,sub-micron sized particles can be used, having a mean particle size ofless than 1μ and can have a mean particle size as small as about 100 nm,10 nm, or less. The coating precursor typically is supplied in an inertliquid or gaseous carrier, such as water, nitrogen, argon, or helium.

[0023] The coating precursor can be co-injected into the detonationchamber (10) together with the detonable mixture (55). Alternatively,the coating precursor can be mixed with the fuel. Optionally, two ormore coating precursors can be alternatively injected into thedetonation chamber (10). The changing from one coating precursor toanother coating precursor can be done at predetermined intervals (e.g.,alternating each detonation, every other detonation, every fifthdetonation, etc.) or can be actuated manually by an operator. Multiplecoating precursors may be used, for example, to create a complexmulti-layered coating material on a substrate using a single coatingapparatus.

[0024] In an alternative embodiment of the invention, a suspension ofthe coating precursor and fuel is injected into the detonation chamber(10). The suspension can be mixed with an oxidizer in the detonationchamber (10) to form a detonable mixture (55). If the fuel is amonopropellant, the suspension can be injected, dispersed, and ignited,e.g., the suspension can be the detonable mixture (55). This embodimentis particularly advantageous for the direct processing of nanoscaleparticles into coatings. The suspension provides for easy injection anduniform dispersion of the coating particles and avoids such problems asclogging of the particle injection line, e.g., resulting from particleagglomeration. Smaller particles also tend to be more reactive and thusdifficult to handle. For example, sub-micron particles of Cu and Ti canself-ignite in air. Mixing the particles with fuel isolates theparticles from atmospheric oxygen, reducing handling hazards.

[0025] In another embodiment of the invention, the coating precursor isinjected in front of (downstream of) the detonable mixture. The coatingparticles can be injected, for example, in an inert gas or in asuspension. A first region of the detonation chamber (10) containing theignition means (30) (e.g., left-hand side of apparatus shown in FIG. 1A)is filled with the detonable mixture. A second region of the detonationchamber (10) closer to the nozzle (100) (right-hand side of theapparatus shown in FIG. 1A) is filled with inert gas or liquid dropletsladen with the coating precursor particles. Following detonation, thedetonation products “push” and accelerate the coating particles, whichare surrounded with the inert gas or liquid. A key advantage is that theinert gas or liquid surrounding the particles helps to avoidcontamination of the particles during coating that can result, forexample, from the particles reacting with oxygen present in thedetonation products. The risk of such contamination generally is moreprevalent when coating with smaller particles. This embodiment isparticularly useful for the coating of nitrates, carbides, and othercompounds that are susceptible to deterioration due to the inclusion ofoxygen or oxides into the coating. Because the particles are insulatedfrom the high temperature detonation products, the particles typicallyare not liquefied or even heated substantially. High particle velocitiesare achieved by expansion into the vacuum or low-pressure environment.

[0026] A suitable ignition means (30), such as a spark plug, is providedin the detonation chamber (10) to ignite the fuel, producing detonationproducts containing the coating precursor. The detonation reactionproduces a brief period of extremely high temperature and high pressureinside the detonation chamber (10). Typical detonation temperatures areon the order of 4000 K, and pressures on the order of 20-30 atmospheresand higher. The period of each detonation typically is less than about10⁻³ sec., preferably less than about 10⁻⁴ sec., and more preferablyless than about 10⁻⁵ sec. As illustrated in FIG. 1B, ignition anddetonation produces a detonation wave front (58) that propagates throughthe detonation chamber (10) toward the nozzle (100). It is preferred tominimize the period of detonation in order to reduce or avoidappreciable local heating of the substrate, and also to permit operationat high frequencies, e.g., as high as 1000 Hz or more. Minimizing theperiod of detonation also avoids or reduces particle grain growth,particularly with nanosized particles.

[0027] In a preferred embodiment illustrated in FIGS. 1A-1C, the pulseddetonation coating apparatus (1) is located in a low-pressure or vacuumchamber (50) that also contains the substrate (200) to be coated. Thevalves, controls, etc. preferably are disposed outside of the vacuumchamber (50) for easy operator access, while the fuel, oxidizer, inertgas/precursor materials, and ignition lines are fed through thelow-pressure or vacuum chamber wall (50) without interfering with thevacuum or low-pressure environment. Suitable low-pressure or vacuummeans (not illustrated) is provided for maintaining low-pressure orvacuum within the low-pressure chamber (50). The term “low pressure” isused herein to refer to pressure substantially lower than atmosphericand typically on the order of 10⁻¹ of atmospheres and lower, often onthe order of 10⁻² to 10⁻³ atmospheres and lower. The term “vacuum” isused herein to refer to pressures of 10⁻⁶ atmospheres and lower.

[0028] As illustrated in FIG. 1C, during the detonation coating cycle,precursor particles are accelerated to high velocities toward thesubstrate (200) via detonation products (60) laden with the coatingprecursor. The detonation products (60) expand from the high-pressure,high-temperature environment of the detonation chamber (10) to thelow-pressure environment of the low-pressure or vacuum chamber (50). Bymaintaining low pressure near the substrate (200), it is possible toproduce high quality coatings (65) using small-micron scale and evennanoscale-sized particles. The particles are effectively accelerated inthe expanding detonation products and do not appreciably decelerate atthe substrate because of the very small drag force in the low-densityand low-pressure environment. In general, the drag force is smaller forsmaller particles. In the low-pressure environment, the characteristicsize of smaller (e.g., nanoscale) particles approaches that of thecollision free path for molecules of the low-pressure carrying gas.Thus, small-micron size particles generally require lower pressures thando smaller, nanoscale particles for the same drag force at the substrateenvironment.

[0029] In the low-pressure chamber (50), the coating precursors areaccelerated to high velocities. The particle velocities can vary over awide range depending on such factors as particle size, detonationpressure, detonation temperature, and the pressure in the low-pressurechamber (50). Typical particle velocities in the practice of the presentinvention are in excess of about 2 km/sec., often 3 km/sec., 4 km/sec.,5 km/sec., or even higher. Because the high temperature detonationproducts heat the coating precursor particles, the coating particlesgenerally are in a liquefied or semi-liquefied state. The low pressureprovides a greater pressure gradient in relation to the detonationpressure, which imparts increased kinetic energy and impact energy tothe coating particles, resulting in high quality coatings.

[0030] The detonation products containing the coating particles aredischarged through the coating nozzle (100) and into contact with thesubstrate (200) to produce a coating (65). The coating nozzle (100)optionally is constructed so that it can be bent and displaced to aplurality of coating positions to permit the apparatus to be used forsuch applications as coating the inside surfaces of pipes and otherdifficult-to-reach portions of substrates. A converging nozzle (100) isshown in FIG. 1A for purposes of illustration. Other nozzleconfigurations can be used. For example, it may be desirable to employ aconverging-diverging nozzle to prevent the detonable mixture and/orcoating precursor material from escaping into the low-pressure chamberprior to detonation.

[0031] A key advantage of the present invention resides in effectivelyremoving or reducing the amount of carrier gases from the detonationproducts as the detonation products are accelerated through thelow-pressure chamber (50) toward the substrate (200), resulting inrelatively low pressure at the substrate surface. This is particularlysignificant for coatings using small-micron and nanosized particles,which are particularly susceptible to being decelerated and divertedaway from the substrate by turbulent gas flow in the vicinity of thesubstrate surface. Such a problem is encountered, for example, inconventional HVOF coating processes.

[0032] The pulsed detonation coating apparatus (1) of the presentinvention can be constructed substantially smaller than conventionalD-guns. For example, the total length of the pulsed detonation coatinggun (1) can be about 50 cm or less. Coating guns (1) can be constructedhaving a total length of about 25 cm or less. It is contemplated thatcoating guns (1) of the present invention can have a length as small asabout 10 cm, 5 cm, or even less.

[0033] Optionally, cooling means (not illustrated) may be provided forcooling the various components of the device (1), for example after eachdetonation. An example of cooling means for a detonation coatingapparatus is shown in U.S. Pat. No. 5,542,606, the disclosure of whichis hereby incorporated by reference. However, it has been found thatcooling the components is unnecessary in most cases due to theintermittent injection of the cool gases between exposures to the hotdetonation products.

[0034] The intermittent detonations advantageously enable the surface ofthe substrate to cool between coated layers. This enables highdeposition rates of coating materials, such as metals or ceramics, ontoa wide variety of substrates, especially those, such as plastic, thathave low melting point surfaces. If necessary, the surface of thesubstrate can be subjected to rapid temperature quenching, for exampleafter each detonation exposure or at other suitable intervals. This canbe done, for example, by intermittently spraying nitrogen onto thesubstrate surface between exposures. Quenching can be also achieved byinjecting liquids such as water, ethyl alcohol, or inert gases such ashelium or argon between the cycles into detonation chamber.

[0035] At particle velocities in excess of 2 km/sec., some particleswill fuse into coatings, even at low temperatures, and create a strongbond with the substrate surface. Excessive heating of the substratesurface can result in previously coated layers can be damaged. Byavoiding overheating of the substrate surface, the intermittentdetonation process of the present invention permits high qualitycoatings to be applied at high coating rates.

[0036] While particular embodiments of the present invention have beendescribed and illustrated, it should be understood that the invention isnot limited thereto since modifications may be made by persons skilledin the art. The present application contemplates any and allmodifications that fall within the spirit and scope of the underlyinginvention disclosed and claimed herein.

What is claimed is:
 1. A method for producing a coating on a substrateusing a pulsed detonation gun, the method comprising: providing a pulseddetonation gun having a detonation chamber, wherein said detonationchamber comprises ignition means and a nozzle for discharging detonationproducts; forming a detonable mixture containing at least one coatingprecursor in said detonation chamber; igniting said detonable mixture toproduce detonation products containing said coating precursor;accelerating said detonation products containing said coating precursorin a low-pressure chamber having a pressure of less than about 10⁻¹atmospheres; and contacting said coating precursor with said substrateto produce a coating on said substrate.
 2. The method of claim 1 whereinsaid at least one coating precursor comprises particles selected fromthe group consisting of metals, cermets, ceramics, and combinationsthereof.
 3. The method of claim 2 wherein said at least one coatingprecursor is a gaseous or liquid metalorganic compound selected from thegroup consisting of silane, disilane, germane, tungsten hexaflurade,trimethylboron, cadmium acetate, magnesium ethoxide, tantalumV-methoxide, tungsten V-ethoxide, zinc naphenate, and zirconiumn-butoxide.
 4. The method of claim 3 where said metalorganic compound ismixed into said detonable mixture.
 5. The method of claim 4 where saidmetalorganic compound is used as fuel for detonation.
 6. The method ofclaim 1 wherein said low-pressure chamber has a pressure of less thanabout 10⁻³ atmospheres.
 7. The method of claim 5 wherein saidlow-pressure chamber has a pressure of less than about 10⁻⁶ atmospheres.8. The method of claim 2 wherein said particles have a mean particlesize of less than about 50μ.
 9. The method of claim 8 wherein said meanparticle size is less than about 10μ.
 10. The method of claim 9 whereinsaid mean particle size is less than about 1μ.
 11. The method of claim10 wherein said mean particle size is less than about 100 nm.
 12. Themethod of claim 11 wherein said mean particle size is less than about 10nm.
 13. The method of claim 1 wherein said step of igniting saiddetonable mixture is intermittently performed at a frequency of fromabout 1 to about 1,000 Hz.
 14. A method for producing a coating on asubstrate using a pulsed detonation gun, the method comprising:providing at least one coating precursor and a detonable mixture;providing a detonation chamber having a first region containing ignitionmeans and a second region having a nozzle for discharging detonationproducts; injecting said detonable mixture into the first region of saiddetonation chamber; injecting said coating precursor into the secondregion of said detonation chamber; igniting said detonable mixture toproduce detonation products, wherein said detonation products displacesaid coating precursor toward said nozzle; accelerating said coatingprecursor in a low-pressure chamber having a pressure of less than about10⁻¹ atmospheres; and contacting said coating precursor with saidsubstrate to produce a coating on said substrate.
 15. The method ofclaim 14 wherein said at least one coating precursor comprises an inertgas or a suspension containing particles having a mean particle size ofless than about 50μ.
 16. The method of claim 15 wherein said meanparticle size is less than about 10μ.
 17. The method of claim 16 whereinsaid mean particle size is less than about 1μ.
 18. The method of claim17 wherein said mean particle size is less than about 100 nm.
 19. Themethod of claim 18 wherein said mean particle size is less than about 10nm.
 20. A method of producing a coating on a substrate using a pulseddetonation coating gun, the method comprising: providing a pulseddetonation gun having a detonation chamber, wherein said detonationchamber comprises ignition means and a nozzle for discharging detonationproducts; providing a suspension of at least one coating precursor in adetonable fuel; injecting said suspension into said detonation chamberand forming a detonable mixture; igniting said detonable mixture toproduce detonation products containing said coating precursor;accelerating said detonation products containing said coating precursorin a low-pressure chamber having a pressure of less than about 10⁻¹atmospheres; and contacting said coating precursor with said substrateto produce a coating on said substrate.
 21. The method of claim 20wherein said at least one coating precursor comprises particles selectedfrom the group consisting of metals, cermets, ceramics, and combinationsthereof, wherein said particles have a mean particle size of less thanabout 50μ.
 22. The method of claim 21 wherein said mean particle size isless than about 10μ.
 23. The method of claim 22 wherein said meanparticle size is less than about 1μ.
 24. The method of claim 23 whereinsaid mean particle size is less than about 100 nm.
 25. The method ofclaim 24 wherein said mean particle size is less than about 10 nm. 26.An apparatus for producing a coating on a substrate by a pulseddetonation coating, the apparatus comprising: a pulsed detonation gunhaving a detonation chamber for receiving a detonable mixture and acoating precursor, wherein said detonation chamber comprises ignitionmeans for initiating said detonable mixture and a nozzle for dischargingdetonation products from said detonation chamber toward a substrate; alow-pressure chamber for accelerating detonation products containingsaid coating precursor discharged from said nozzle; and low-pressuremeans for maintaining a pressure less than about 10⁻¹ atmospheres insaid low-pressure chamber.
 27. The apparatus of claim 26 wherein saidignition means intermittently ignites said detonable mixture at afrequency of from about 1 to about 1,000 Hz.
 28. The apparatus of claim26 wherein said low-pressure means maintains a pressure not exceedingabout 10⁻³ atmospheres in said low-pressure chamber.
 29. The apparatusof claim 28 wherein said low-pressure means maintains a pressure notexceeding about 10⁻⁶ atmospheres in said low-pressure chamber.
 30. Theapparatus of claim 26 wherein said nozzle is displaceable to a pluralityof coating positions.
 31. The apparatus of claim 26 wherein said nozzlecomprises a converging nozzle.
 32. The apparatus of claim 26 whereinsaid nozzle comprises a converging-diverging nozzle.
 33. The apparatusof claim 26 wherein said pulsed detonation gun has a length of about 50cm or less.
 34. The apparatus of claim 33 wherein the length of saidpulsed detonation gun is about 25 cm or less.
 35. The apparatus of claim34 wherein the length of said pulsed detonation gun is about 10 cm orless.